ORIGINAL PAPER High-pressure electronic absorption spectroscopy of natural and synthetic Cr 3+ -bearing clinopyroxenes M. N. Taran • H. Ohashi • K. Langer • A. A. Vishnevskyy Received: 7 June 2010 / Accepted: 29 October 2010 / Published online: 17 November 2010 Ó Springer-Verlag 2010 Abstract Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi 2 O 6 and LiCrSi 2 O 6 clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similar- ities reflect the fact that in all cases Cr 3? enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe 2? (M1, M2) and intervalence Fe 2? /Fe 3? charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr 3? in the crystal structures. The positions of the spin-allowed bands of Cr 3? , especially of the low energy one caused by the electronic 4 A 2g ? 2 T 1g transi- tion, are found to be in accordance with mean M1–O distances. The local relaxation parameter e calculated for lim Cr 3? ? 0 from the spectra and interatomic Cr  O h i and Mg  O h i distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around the ‘‘guest’’ ion, Cr 3? , deviates greatly from the ‘‘diffraction’’ value, e = 0, than in any other known Cr 3? -bearing systems studied so far. Under pressure the spin-allowed bands of Cr 3? shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while the spin-forbidden absorption lines remain practically un- shifted, but also undergo a strong weakening. There is no evident dependence of the Racah parameter B of Cr 3? reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded as practically constant. The values of CrO 6 octahedral modulus, k loc poly , derived from high-pres- sure spectra of natural chromium diopside and synthetic NaCrSi 2 O 6 kosmochlor are very close, *203 and *196 GPa, respectively, being, however, nearly twice higher than that of MgO 6 octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs (2008). Such a strong stiffening of the structural octahedron, i.e. twice higher value of k loc Cr 3þ comparing with that of k loc Mg 2þ , may be caused by simultaneous substitution of Ca 2? by larger Na ? in the neighboring M2 sites at so-called jadeite-coupled substitution Mg 2? ? Ca 2? ? Cr 3? ? Na ? . It is also remarkable that the values of CrO 6 octahedral modulus of NaCrSi 2 O 6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high- pressure X-ray structural refinement by Origlieri et al. (2003). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the com- pressibility of M1- and M2-sites, our k loc Cr 3þ -value, *196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa. Keywords Clinopyroxenes  Cr 3? electronic absorption spectra  Pressure behavior M. N. Taran (&)  A. A. Vishnevskyy Institute of Geochemistry, Mineralogy and Ore Formation, National Academy of Science of Ukraine, Palladin Ave., 34, Kyiv 142 03680, Ukraine e-mail: m_taran@hotmail.com H. Ohashi HASHI Institute for Silicate Science, Nishinakanobu 1-9-25, Shinagawa, Tokyo 142-0054, Japan e-mail: haruohashi@hotmail.com K. Langer Institute of Applied Geosciences Technical University Berlin, Ernst Reuter-Platz 1, 10587 Berlin, Germany 123 Phys Chem Minerals (2011) 38:345–356 DOI 10.1007/s00269-010-0408-x